Electromagnetic brake and drive force distributing apparatus for vehicle using the electromagnetic brake

ABSTRACT

An electromagnetic brake interposed between a fixed housing and a rotating member. The electromagnetic brake includes a multiplate brake mechanism, a ringlike core member fixed in the fixed housing, an annular exciting coil accommodated in an annular groove formed on the core member, and a ringlike armature member opposed to the annular groove of the core member and having an outer diameter larger than the outer diameter of the core member. The electromagnetic brake further includes an annular pressure plate axially and movably mounted on the fixed housing at one end portion of the multiplate brake mechanism adjacent to the core member, and a cylindrical pressure member provided so as to surround the outer circumferential surface of the core member and be movable in a direction of pressing the multiplate brake mechanism as being guided by the core member. One end of the cylindrical pressure member is fixed to an outer circumferential portion of the armature member, and the other end is engaged with the annular pressure plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electromagnetic brake and a driveforce distributing apparatus for a vehicle using the electromagneticbrake.

2. Description of the Related Art

A differential is located in a power train of a vehicle to maintaintorque distribution between right and left wheels of the vehicle suchthat torque is equally divided between the right and left wheels and torotate the outside wheel faster than the inside wheel during cornering,thereby reliably obtain smooth cornering. While the primary role of thedifferential is to obtain smooth cornering as mentioned above, there isa case that one of the right and left wheels may be caught to slip in amuddy place during rough-road running.

In this case, the resistance from the road to the wheel caught to slipin the muddy place is small, so that torque is almost transmitted tothis slipping wheel and hardly transmitted to the other wheel. As aresult, there is a problem that the slipping wheel cannot escape fromthe muddy place for lack of the drive force for driving the wheels. Thisis a problem in a defect inherent in a general differential. Known is aspecial type of differential having a differential motion limitingmechanism capable of compensating for the above defect inherent in ageneral differential. This type of differential is referred to as alimited slip differential (LSD).

A planetary gear type differential is generally known in the art, forexample, such a planetary gear type differential gear assembly having alimited slip differential mechanism composed of an electromagneticclutch and a multiplate clutch is disclosed in Japanese Patent Laid-openNo. Hei 6-33997. In this differential gear assembly, an attraction forcebetween a solenoid and an armature forming the electromagnetic clutch isapplied to the multiplate clutch to press it and selectively control anengaging force generated in the multiplate clutch. A connecting memberconsisting of a plurality of bars is located between a pressure plate ofthe multiplate clutch and the armature. That is, one end of each bar ofthe connecting member is fixed to the pressure plate of the multiplateclutch, and the other end abuts against an inner circumferential portionof the armature when the solenoid is operated.

In the conventional differential gear assembly mentioned above, theplural bars fixed to the pressure plate extend in a directionsubstantially perpendicular to the pressure plate. Accordingly, in thecase that any of these bars are inclined to the pressure plate, there isa problem that a pressing force of the armature attracted by thesolenoid to press the pressure plate of the multiplate clutch may not beuniformly transmitted to the pressure plate. Further, in theconventional differential gear assembly described in the abovepublication, the electromagnetic clutch controls the engaging force ofthe multiplate clutch, so that the plural bars as pressure members arelocated so as to correspond to the inner circumferential portion of thearmature. However, in a multiplate brake structure having a plurality ofbrake plates and a plurality of brake discs, these brake plates andbrake discs are generally located so as to correspond to an outercircumferential portion of the armature from the viewpoint of thestructure. Accordingly, it is difficult that the conventional structuredescribed in the above publication such that the multiplate clutch isoperatively connected to the armature at its inner circumferentialportion is applied to the multiplate brake structure without anychanges.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anelectromagnetic brake which can achieve accurate positioning between anarmature and a core member in the radial direction and can accuratelycontrol an engaging force generated in a multiplate brake mechanism.

It is another object of the present invention to provide a drive forcedistributing apparatus for a vehicle including the above electromagneticbrake which can arbitrarily distribute a drive force between right andleft drive wheels of the vehicle.

In accordance with an aspect of the present invention, there is providedan electromagnetic brake interposed between a fixed housing and arotating member at least partially accommodated in said fixed housing.The electromagnetic brake includes a multiplate brake mechanism having aplurality of brake plates mounted on said fixed housing and a pluralityof brake discs mounted on said rotating member so as to be arranged inalternate relationship with said brake plates; a ringlike core memberfixed in said fixed housing, said core member having an annular grooveand a first outer diameter; an annular exciting coil accommodated insaid annular groove of said core member; and a ringlike armature memberarranged in opposed relationship with said annular groove of said coremember, said armature member having a second outer diameter larger thansaid first outer diameter. The electromagnetic brake further includes acylindrical pressure member provided so as to surround the outercircumferential surface of said core member and be movable in adirection of pressing said multiplate brake mechanism as being guided bysaid core member, said pressure member having a first end fixed to anouter circumferential portion of said armature member and a second endengaged with said multiplate brake mechanism.

With this configuration, accurate positioning between the armaturemember and the core member in the radial direction can be achieved bythe cylindrical pressure member. Further, since the cylindrical pressuremember is fixed to the armature member, inclination of the armaturemember with respect to the axial direction can be prevented, and a gapdefined between the armature member and the core member can beaccurately uniformed in the radial direction, thereby allowing accuratecontrol of an engaging force generated in the multiplate brakemechanism. Accordingly, a braking function can be achieved withoutdirect metallic contact, and an attraction force between the core memberand the armature member can be efficiently transmitted to the multiplatebrake mechanism. Further, the rigidity of the components of theelectromagnetic brake in relation to the transmission of the attractionforce is considered to minimize the elastic deformation of thesecomponents, thereby reducing the hysteresis and stably generating theattraction force.

Preferably, said core member has a plurality of fastening portionsadapted to be fastened to said fixed housing, said fastening portionsprojecting radially outward from the outer circumference surface of saidcore member, and said pressure member has a plurality of cutouts forallowing insertion of said fastening portions of said core member. Withthis configuration, accurate positioning of the core member in theradial and axial directions with respect to the fixed housing can beachieved by the fastening portions. Furthermore, by forming the cutoutsin the cylindrical pressure member, the pressure member fixed to thearmature member can be fastened to the fixed housing in the conditionwhere the pressure member is fitted with the core by inserting thefastening portions of the core member into the cutouts of the pressuremember. Thus, the assembly of the electromagnetic brake can be easilyperformed.

Preferably, said core member has an inner circumferential portion and anouter circumferential portion divided from each other by said annulargroove, the sectional area of said inner circumferential portion beingsubstantially equal to that of said outer circumferential portion. Withthis configuration, the attraction force can be uniformed in the radialdirection of the core member. More preferably, the inner circumferentialsurface of said pressure member is formed with a plurality ofprojections spaced apart from each other in the circumferentialdirection, and said pressure member is movable in said pressingdirection so that said projections of said pressure member is in slidingcontact with the outer circumferential surface of said core member. Withthis configuration, the cylindrical pressure member can be moved axiallystraight as being guided by the core member when the armature member isattracted to the core member. Accordingly, a pressing force uniform inthe circumferential direction can be applied to the multiplate brakemechanism.

Further, the projections for ensuring the accuracy of alignment of thepressure member are formed on a part of the inner circumferentialsurface of the pressure member rather than the whole thereof, therebyallowing simplification of the structure and a reduction in frictionduring axial movement of the pressure member. Further, since thepressure member is provided so as to surround the outer circumferentialsurface of the core member, the second end of the pressure member canpress the brake plates and the brake discs of the multiplate brakemechanism at a substantially central position in the effective radius.Accordingly, a uniform pressing force can be applied to the brake platesand the brake discs of the multiplate brake mechanism.

In accordance with another aspect of the present invention, there isprovided a drive force distributing apparatus for a vehicle having apair of drive wheels which includes a fixed housing; a first axleconnected to one of said drive wheels; a second axle connected to theother drive wheel; an input shaft rotatably mounted in said fixedhousing and connected to a drive source; a first planetary gear assemblyhaving a first ring gear operatively connected to said input shaft, afirst planetary carrier fixed to said first axle, a first sun gearrotatably mounted on said first axle, and a first planet gear carried bysaid first planetary carrier so as to mesh with both said first ringgear and said first sun gear; a second planetary gear assembly having asecond ring gear operatively connected to said input shaft, a secondplanetary carrier fixed to said second axle, a second sun gear rotatablymounted on said second axle, and a second planet gear carried by saidsecond planetary carrier so as to mesh with both said second ring gearand said second sun gear; a first multiplate brake mechanism interposedbetween said fixed housing and said first sun gear; a firstelectromagnetic brake for controllably operating said first multiplatebrake mechanism; a second muitiplate brake mechanism interposed betweensaid fixed housing and said second sun gear; and a secondelectromagnetic brake for controllably operating said second multiplatebrake mechanism; a drive force from said input shaft being distributedbetween said first axle and said second axle by operating said firstelectromagnetic brake and said second electromagnetic brake.

In accordance with a further aspect of the present invention, there isprovided a drive force distributing apparatus for a four-wheel drivevehicle having a pair of first drive wheels and a pair of second drivewheels, which includes a fixed housing; a first axle connected to one ofsaid first drive wheels; a second axle connected to the other firstdrive wheel; an input shaft rotatably mounted in said fixed housing andconnected to a drive source; a first planetary gear assembly having afirst ring gear operatively connected to said input shaft, a firstplanetary carrier fixed to said first axle, a first sun gear rotatablymounted on said first axle, and a first planet gear carried by saidfirst planetary carrier so as to mesh with both said first ring gear andsaid first sun gear; a second planetary gear assembly having a secondring gear operatively connected to said input shaft, a second planetarycarrier fixed to said second axle, a second sun gear rotatably mountedon said second axle, and a second planet gear carried by said secondplanetary carrier so as to mesh with both said second ring gear and saidsecond sun gear; a first multiplate brake mechanism interposed betweensaid fixed housing and said first sun gear; a first electromagneticbrake for controllably operating said first multiplate brake mechanism;a second multiplate brake mechanism interposed between said fixedhousing and said second sun gear; and a second electromagnetic brake forcontrollably operating said second multiplate brake mechanism; a driveforce from said input shaft being distributed among said first axle,said second axle, and said second drive wheels by operating said firstelectromagnetic brake and said second electromagnetic brake.

Preferably, said first electromagnetic brake includes a first ringlikecore member fixed in said fixed housing, said first core member having afirst annular groove and a first outer diameter; a first annularexciting coil accommodated in said first annular groove of said firstcore member; a first ringlike armature member arranged in opposedrelationship with said first annular groove of said first core member,said first armature member having a second outer diameter larger thansaid first outer diameter; a first annular pressure plate axially andmovably mounted on any one of said fixed housing and said first axle atone end portion of said first multiplate brake mechanism adjacent tosaid first core member; and a first cylindrical pressure member providedso as to surround the outer circumferential surface of said first coremember and be movable in a direction of pressing said first multiplatebrake mechanism as being guided by said first core member, said firstpressure member having a first end fixed to an outer circumferentialportion of said first armature member and a second end engaged with saidfirst annular pressure plate.

Preferably, said second electromagnetic brake includes a second ringlikecore member fixed in said fixed housing, said second core member havinga second annular groove and a third outer diameter; a second annularexciting coil accommodated in said second annular groove of said secondcore member; a second ring-like armature member arranged in opposedrelationship with said second annular groove of said second core member,said second armature member having a fourth outer diameter larger thansaid third outer diameter; a second annular pressure plate axially andmovably mounted on any one of said fixed housing and said second axle atone end portion of said second multiplate brake mechanism adjacent tosaid second core member; and a second cylindrical pressure memberprovided so as to surround the outer circumferential surface of saidsecond core member and be movable in a direction of pressing said secondmultiplate brake mechanism as being guided by said second core member,said second pressure member having a third end fixed to an outercircumferential portion of said second armature member and a fourth endengaged with said second annular pressure plate.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing the configuration of a FF(front-engine front-drive) vehicle on which the drive force distributingapparatus according to the present invention is mounted;

FIG. 2 is a schematic plan view showing the configuration of afour-wheel drive vehicle on which the drive force distributing apparatusaccording to the present invention is mounted;

FIG. 3 is a sectional view of the drive force distributing apparatusshown in FIG. 2;

FIG. 4 is an elevational view of a side housing;

FIG. 5 is a right side view of the left side housing shown in FIG. 4;

FIG. 6A is an elevational view of an annular pressure plate;

FIG. 6B is a cross section taken along the line 6B—6B in FIG. 6A;

FIG. 7A is an elevational view of a ringlike core member;

FIG. 7B is a cross section taken along the line 7B—7B in FIG. 7A;

FIG. 8 is a sectional view of a ringlike armature member;

FIG. 9A is an elevational view of a cylindrical pressure member;

FIG. 9B is a cross section taken along the line 9B—9B in FIG. 9A; and

FIG. 9C is an enlarged view of an encircled portion 125 shown in FIG.9A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be described indetail with reference to the drawings. FIG. 1 is a schematic plan viewshowing the configuration of a front-engine front-drive (FF) vehicle towhich a drive force distributing apparatus 6 having the electromagneticbrake of the present invention is applied. A drive force from an engine2 is transmitted through a transmission 4 to the drive forcedistributing apparatus 6. The drive force transmitted is distributedbetween a left front axle 8 and a right front axle 10 by the drive forcedistributing apparatus 6. The drive force thus distributed drives a leftfront wheel 12 mounted on the left front axle 8 and a right front wheel14 mounted on the right front axle 10.

FIG. 2 is a schematic plan view showing the configuration of afour-wheel drive vehicle to which a drive force distributing apparatus20 having the electromagnetic brake of the present invention is applied.A drive force from an engine 2 drives left and right front wheels 12 and14 through a transmission 4 and left and right front axles 8 and 10. Thedrive force is also transmitted through a propeller shaft 18 to thedrive force distributing apparatus 20 having substantially the sameconfiguration as that of the drive force distributing apparatus 6 shownin FIG. 1. The drive force transmitted to the drive force distributingapparatus 20 is distributed between a left rear axle 22 and a right rearaxle 24 at a given ratio. The drive force thus distributed drives a leftrear wheel 26 mounted on the left rear axle 22 and a right rear wheel 28mounted on the right rear axle 24. As will be hereinafter described indetail, the drive force distributing apparatus 20 incorporates a pair ofelectromagnetic brakes. By controlling braking forces of theelectromagnetic brakes, the drive force from the propeller shaft 18 canbe arbitrarily distributed between the rear wheels 26 and 28. Further,in the case of idling the rear wheels 26 and 28, the drive force fromthe engine 2 can be entirely supplied to the front wheels 12 and 14. Inthis case, this four-wheel drive vehicle operates as an FF vehicle.

Referring to FIG. 3, there is shown a sectional view of the drive forcedistributing apparatus 20. Reference numeral 30 denotes a fixed housing.The fixed housing 30 is composed of a central housing 30 a, a left sidehousing 30 b, a right side housing 30 c, and an intermediate housing 30d. The left side housing 30 b and the intermediate housing 30 d arefastened to the central housing 30 a by screws 32 and 34. The right sidehousing 30 c is fastened to the central housing 30 a by screws 36. FIG.4 is an elevational view of the left side housing 30 b, and FIG. 5 is aright side view of FIG. 4.

The left rear axle 22 is rotatably supported in the housing 30 by a pairof bearings 38 and 40. Similarly, the right rear axle 24 is rotatablysupported in the housing 30 by a pair of bearings 42 and 44. The leftrear axle 22 is connected to the left rear wheel 26, and the right rearaxle 24 is connected to the right rear wheel 28. Reference numeral 46denotes a companion flange, which is fastened to the propeller shaft 18shown in FIG. 2 by screws (not shown). An input shaft 50 is rotatablysupported in the housing 30 by a pair of needle bearings 52 and 54. Theinput shaft 50 is connected at its front end to the companion flange 46by splines 48. The input shaft 50 is formed at its rear end with a bevelgear 56.

A planetary gear assembly 58A is interposed between the input shaft 50and the left rear axle 22, and a planetary gear assembly 58B isinterposed between the input shaft 50 and the right rear axle 24. Theplanetary gear assembly 58A has substantially same structure as that ofthe planetary gear assembly 58B, so like parts are denoted by the samereference numerals and only the planetary gear assembly 58A will now beprimarily described.

Reference numeral 60 denotes an input ring gear of the planetary gearassembly 58A. The input ring gear 60 is formed at its right end with abevel gear 62. The bevel gear 62 of the input ring gear 60 meshes withthe bevel gear 56 of the input shaft 50. The planetary gear assembly 58Bhas a ring gear 60′ connected to the ring gear 60 of the planetary gearassembly 58A by splines 63. Accordingly, the ring gear 60′ of theplanetary gear assembly 58B is rotationally driven by the input shaft 50through the ring gear 60 of the planetary gear assembly 58A.

The planetary gear assembly 58A further includes a planetary carrier 64,a sun gear 68, and a plurality of planet gears 72 (only one of whichbeing shown). The planetary carrier 64 is fixed to the left rear axle 22by splines 66. The sun gear 68 is rotatably mounted on the left rearaxle 22 by a bearing 70. Each planet gear 72 is carried by the planetarycarrier 64 and meshes with both the sun gear 68 and the ring gear 60.Reference numeral 74 denotes a wet type multiplate brake mechanism. Thewet type multiplate brake mechanism 74 includes a plurality of brakeplates 76 mounted on the housing 30 and a plurality of brake discs 78mounted on the sun gear 68. The brake plates 76 and the brake discs 78are alternately arranged.

Each brake plate 76 is mounted on the housing 30 so as to be axiallymovable and unrotatable, and each brake disc 78 is mounted on the sungear 68 so as to be axially movable and unrotatable. A snap ring 80 ismounted on the housing 30 to axially position one end (the right end) ofthe multiplate brake mechanism 74. Fine adjustment of this positioningis made by controlling the thickness of a shim 82 located axiallyadjacent to the snap ring 80.

An annular pressure plate 84 is provided at the other end (the left end)of the multiplate brake mechanism 74. As shown in FIG. 6A, the annularpressure plate 84 has a plurality of projections 86 spaced apart fromeach other in the circumferential direction. These projections 86 areinserted in axial grooves formed on the inner wall of the housing 30, sothat the annular pressure plate 84 is mounted on the housing 30 so as tobe axially movable and unrotatable. As best shown in FIG. 6B, theannular pressure plate 84 is formed at its outer circumferential portionwith an annular groove 88 for insertion of a cylindrical pressure memberto be hereinafter described. The annular pressure plate 84 may bemounted on the sun gear 68.

Reference numeral 90 denotes a ringlike core member, which has a firstouter diameter and an annular groove 96 having a rectangular crosssection. As shown in FIG. 7A, the ringlike core member 90 has a centralhole 91 and a pair of fastening portions 94. Each fastening portion 94is formed with a hole 95 for insertion of a screw 92 (see FIG. 3). Asbest shown in FIG. 7B, an exciting coil 98 is accommodated in theannular groove 96. The core member 90 is divided into an innercircumferential portion 90 a and an outer circumferential portion 90 bby the annular groove 96. The sectional area of the innercircumferential portion 90 a is substantially equal to that of the outercircumferential portion 90 b.

As shown in FIG. 7A, the core member 90 has four projections 102, arecess 104 for insertion of an exciting coil terminal 108 (see FIG. 3),and a recess 106 for insertion of a search coil terminal (not shown). Asshown in FIG. 3, a search coil 100 is mounted in the annular groove 96adjacent to the exciting coil 98. The search coil 100 is provided todetect the intensity of magnetic flux in passing a current through theexciting coil 98 and control a coil current supplied to the excitingcoil 98 according to the detected intensity of magnetic flux.

As shown in FIG. 5, the left side housing 30 b has a central hole 39 anda pair of mounting portions 114. Each mounting portion 114 is formedwith a tapped hole 115. The left side housing 30 b further has anannular abutting portion 116. The core member 90 is fixed to the leftside housing 30 b by making the projections 102 of the core member 90abut against the annular abutting portion 116 of the left side housing30 b, making the fastening portions 94 of the core member 90 abutagainst the mounting portions 114 of the left side housing 30 b, andinserting the screws 92 through the holes 95 of the fastening portions94 to threadedly engage the screws 92 into the tapped holes 115 of themounting portions 114.

A ringlike armature member 110 formed of a magnetic material is locatedso as to be opposed to the annular groove 96 of the core member 90. Asshown in FIG. 8, the armature member 110 has a second outer diameterlarger than the first outer diameter of the core member 90, a centralhole 111, and an annular mounting groove 112 formed at an outercircumferential portion. The armature member 110 is tapered from itsinner circumference toward its outer circumference as viewed in crosssection, so as to uniform a magnetic path in passing a current throughthe exciting coil 98 and to reduce the weight.

A cylindrical pressure member 120 has a first end (left end)press-fitted with the annular mounting groove 112 of the armature member110, and a second end (right end) inserted in the annular groove 88 ofthe annular pressure plate 84. In inserting the second end of thecylindrical pressure member 120 into the annular groove 88 of theannular pressure plate 84, the outer circumference of the cylindricalpressure member 120 is positioned with respect to the annular groove 88.That is, the second end of the cylindrical pressure member 120 isinserted into the annular groove 88 of the annular pressure plate 84 inthe condition where the inner circumference of the cylindrical pressuremember 120 is loosely fitted with the inner circumference of the annulargroove 88 and the outer circumference of the cylindrical pressure member120 is closely fitted with the outer circumference of the annular groove88.

As shown in FIGS. 9A and 9B, the cylindrical pressure member 120 has apair of cutouts 122 for insertion of the pair of fastening portions 94of the core member 90 and four cutouts 124 for insertion of the fourprojections 102 of the core member 90. The inner circumferential surfaceof the cylindrical pressure member 120 is formed with six projections126 spaced apart from each other in the circumferential direction.Accordingly, the cylindrical pressure member 120 is movable in itspressing direction (axial direction) in the condition where theprojections 126 are in sliding contact with the outer circumferentialsurface of the core member 90.

An electromagnetic brake 130A including the multiplate brake mechanism74 is assembled by first press-fitting the first end (left end) of thecylindrical pressure member 120 into the annular mounting groove 112 ofthe armature member 110, next covering the ring-like core member 90 withthe cylindrical pressure member 120 fixed to the armature member 110,next inserting the second end (right end) of the cylindrical pressuremember 120 into the annular groove 88 of the annular pressure plate 84,and finally fastening the ringlike core member 90 at the pair offastening portions 94 to the housing 30.

As mentioned above, the sectional area of the inner circumferentialportion 90 a of the core member 90 is substantially equal to that of theouter circumferential portion 90 b of the core member 90. To this end,the width of the inner circumferential portion 90 a is set larger thanthat of the outer circumferential portion 90 b as viewed in the crosssection perpendicular to the axial direction. Furthermore, the ringlikearmature member 110 is tapered from the inner circumference toward theouter circumference, so as to uniform a magnetic path in passing acurrent through the exciting coil 98. With this configuration, thearmature member 110 can be attracted by a uniform force over the radiusthereof in passing a current through the exciting coil 98. That is, byuniforming the magnetic path, the armature member 110 can be preventedfrom being inclined with respect to the axial direction, and an engagingforce of the multiplate brake mechanism 74 in the electromagnetic brake130A can therefore be accurately controlled.

When a current is passed through the exciting coil 98, a predeterminedgap is defined between the core member 90 and the armature member 110,thereby preventing metallic contact between the core member 90 and thearmature member 110. The axial positioning of the armature member 110 inthe condition where the armature member 110 is attracted to the coremember 90 by passing a current through the exciting coil 98 isdetermined by the mounting portions 114 of the left side housing 30 bfor fastening the core member 90 to the left side housing 30 b and bythe position of the snap ring 80 provided at the right end of themultiplate brake mechanism 74 and fixed to the left side housing 30 b.

The fine adjustment of this axial positioning is made by controlling thethickness of the shim 82 located adjacent to the snap ring 80 to therebycontrol the accuracy of the gap between the core member 90 and thearmature member 110. While the left planetary gear assembly 58A and theleft electromagnetic brake 130A have been described, the right planetarygear assembly 58B and the right electromagnetic brake 130B aresubstantially the same in structure as the left planetary gear assembly58A and the left electromagnetic brake 130A, respectively, so thedescription of the right planetary gear assembly 58B and the rightelectromagnetic brake 130B will be omitted herein.

According to the electromagnetic brake 130A in this preferredembodiment, the cylindrical pressure member 120 is located around theouter circumferential surface of the ringlike core member 90, so thatthe right end of the pressure member 120 can press the plural brakeplates 76 and the plural brake discs 78 forming the multiplate brakemechanism 74 at their substantially central portions in respect of theeffective radius of each element. Accordingly, a uniform pressing forceto the multiplate brake mechanism 74 can be obtained with no radialdeviation. Further, since the pressing force of the cylindrical pressuremember 120 to the multiplate brake mechanism 74 is applied axiallystraight as being guided by the core member 90, a reduction in controlaccuracy of braking engagement due to deflection of the cylindricalpressure member 120 can be suppressed.

A brake plate with a facing known in the art can be used without anychanges as each brake plate 76 of the multiplate brake mechanism 74,thereby preventing seizure and judder occurring between metallic platesforming the multiplate brake mechanism 76 during the operation of theelectromagnetic brake 130A. Since an air gap is defined between theexciting coil 98 and the armature member 110, no residual magnetism isgenerated in a magnetic path in attracting the armature member 110,thereby improving the stability of control of an attraction force to thearmature member 110 and eliminating the need for any parts for cancelingan attraction force due to residual magnetism. Furthermore, it ispossible to improve the falling response in turning off an electricalsignal to the electromagnetic brake 130A and the rising response inturning on an electrical signal to the electromagnetic brake 130A.Furthermore, since the electromagnetic brake 130A is simple instructure, the hysteresis can be reduced.

The operation of this preferred embodiment will now be described. Whenboth the electromagnetic brakes 130A and 130B are in an off state withno currents being passed through the exciting coils 98 of theelectromagnetic brakes 130A and 130B, both the multiplate brakemechanism 74 are in a disengaged state, so that the sun gears 68 of theplanetary gear assemblies 58A and 58B idly rotate about the left andright rear axles 22 and 24, respectively. Accordingly, the drive force(torque) from the input shaft 50 is not transmitted to the rear axles 22and 24. In this case, the rear wheels 26 and 28 idly rotate and thedrive force is entirely transmitted to the front wheels 12 and 14, sothat the four-wheel drive vehicle shown in FIG. 3 operates in atwo-wheel drive mode (FF vehicle).

When a predetermined amount of current is passed through the excitingcoils 98 of the electromagnetic brakes 130A and 130B to completelyengage both the multiplate brake mechanisms 74 through the cylindricalpressure members 120 of the electromagnetic brakes 130A and 130B, thesun gears 68 of the planetary gear assemblies 58A and 58B are fixed tothe left and right rear axles 22 and 24, respectively. Accordingly, thedrive force from the input shaft 50 is equally divided between the rearaxles 22 and 24 and transmitted thereto. As a result, the four-wheeldrive vehicle shown in FIG. 2 operates in a four-wheel drive mode to runstraight. In the case of a front-engine rear-drive (FR) vehicle, theentirety of the drive force is equally divided between the rear wheels,and this vehicle runs straight.

In cornering or escaping from a muddy place, the amperages of thecurrents passing through the exciting coils 98 of the electromagneticbrakes 130A and 30B are controlled to thereby arbitrarily distribute thedrive force from the input shaft 50 between the rear axles 22 and 24, sothat optimum cornering control and/or easy escape from the muddy placecan be realized.

While the drive force distributing apparatus 20 provided in relation tothe rear axles 22 and 24 has been described above with reference toFIGS. 2 to 9C, the drive force distributing apparatus 6 provided inrelation to the front axles 8 and 10 as shown in FIG. 1 also has similaroperations and effects. Further, while the drive force distributingapparatus 20 is provided in relation to the rear axles 22 and 24 of thefour-wheel drive vehicle in this preferred embodiment, the apparatus 20may be provided in relation to the rear axles of an FR vehicle. Further,while the electromagnetic brake of the present invention is applied tothe drive force distributing apparatus 20 in this preferred embodiment,the present invention is not limited to this preferred embodiment, butmay be applied to any mechanisms or apparatuses having anelectromagnetic brake interposed between a fixed housing and a rotatingmember.

While the preferred embodiments of the present invention have beendescribed using the specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the followingclaims.

1. An electromagnetic brake interposed between a fixed housing and arotating member at least partially accommodated in said fixed housing,comprising: a multiplate brake mechanism having a plurality of brakeplates mounted on said fixed housing and a plurality of brake discsmounted on said rotating member so as to be arranged in alternaterelationship with said brake plates; a ringlike core member fixed insaid fixed housing, said core member having an annular groove and afirst outer diameter; an annular exciting coil accommodated in saidannular groove of said core member; a ringlike armature member arrangedin opposed relationship with said annular groove of said core member,said armature member having a second outer diameter larger than saidfirst outer diameter; and a cylindrical pressure member provided so asto surround the outer circumferential surface of said core member and bemovable in a direction of pressing said multiplate brake mechanism asbeing guided by said core member, said pressure member having a firstend fixed to an outer circumferential portion of said armature memberand a second end engaged with said multiplate brake mechanism.
 2. Anelectromagnetic brake according to claim 1, wherein said core member hasa plurality of fastening portions adapted to be fastened to said fixedhousing, said fastening portions projecting radially outward from theouter circumference surface of said core member, and said pressuremember has a plurality of cutouts for allowing insertion of saidfastening portions of said core member.
 3. An electromagnetic brakeaccording to claim 1, wherein said core member has an innercircumferential portion and an outer circumferential portion dividedfrom each other by said annular groove, the sectional area of said innercircumferential portion being substantially equal to that of said outercircumferential portion.
 4. An electromagnetic brake according to claim1, wherein the inner circumferential surface of said pressure member isformed with a plurality of projections spaced apart from each other inthe circumferential direction, and said pressure member is movable insaid pressing direction so that said projections of said pressure memberis in sliding contact with the outer circumferential surface of saidcore member.
 5. An electromagnetic brake according to claim 1, whereinsaid cylindrical pressure member presses the plural brake plates and theplural brake discs at their substantially central portions in respect ofthe effective radius of the plural brake plates and the plural brakediscs.
 6. An electromagnetic brake according to claim 1, wherein thepressing force of said cylindrical pressure member to said multiplebrake mechanism is applied axially straight as being guided by said coremember.